1. Field of the Invention
This invention relates to data transmission.
2. Description of the Prior Art
Direct Stream Digital (DSD) is a high-resolution single-bit audio coding system used for the so-called Super Audio CD consumer disc format. DSD was developed with a view to producing audio signals comparable to those reproduced from the best analogue formats. DSD signals can produce a frequency response from DC to 100 kHz and have a dynamic range of greater than 120 dB across the audio band.
DSD makes use of 1-bit digital audio and requires a high frequency audio sample clock at 64 Fs=2.8224 MHz (the sample clock of standard PCM systems (Fs) is 44.1 kHz). This high frequency sample clock is transmitted along with the data to facilitate accurate signal reconstruction at the receiving end. Furthermore each channel of 64 Fs DSD audio requires a transmission bandwidth of 2.8224 Mbit/s.
Several known audio networking systems make use of Ethernet to transmit high bandwidth audio-data between a network of audio processing devices. For example the “Magic” system proprietary to Gibson makes use of the Ethernet Media Access Control MAC layer (i.e. physical layer and data link layer) to transmit audio data at a fixed audio sampling frequency of 48 kHz using one Ethernet frame per sample period. The CobraNet audio networking system proprietary to Peak Audio also uses the Ethernet MAC layer to transmit uncompressed digital audio data between networked devices. The CobraNet system uses a 48 kHz sampling rate and allows for transmission of 20-bit and 24-bit audio data. However, none of these known systems provides an interconnection suitable for linking DSD audio devices. This is because Ethernet frame timing is completely unsuitable for transmitting a 2.8224 MHz DSD sample clock.
Copending application Ser. Nos. 10/620,671 (now U.S. Pat. No. 7,084,927, issued Aug. 1, 2006) and 10/803,621 (now U.S. Pat. No. 7,499,500, issued Mar. 3, 2009) relate to techniques for carrying such bitstreams over the physical layer of an Ethernet-type interface. The systems described in these two copending applications aim to provide circuit-switched interconnections between large-scale multi-track production equipment for DSD audio such as multi-channel ADC/DACs, DSD mixers and multi-channel DSD recorders.
So, in the systems of the two copending applications techniques are provided which are based on the direct use of the Ethernet physical layer interface. This can be shown to provide effective and low-latency circuit-switched links at audio data rates of the order of 64 Fs, such systems also being useful for the transmission of other streamed digital signals such as PCM audio signals. Auxiliary data channels can also be handled by the same means. However, as this is achieved by deliberately avoiding the use of higher levels of the Ethernet protocol, there is no convenient way of providing packet-switched auxiliary data routing within such a system.